TweetFollow Us on Twitter

January 92 - The Soup Kitchen - Extending C++ with C++

The Soup Kitchen - Extending C++ with C++

Eric M. Berdahl

This month I'll look at C++ from a slightly different perspective. One of my central themes has been object code compatibility with Pascal and MacApp. But this will not be an issue today. Instead, I'll be looking at tools intended to make the C++ programmer's life easier by extending the language with "pure" C++ classes.

THE MEANING OF "EXTENDABLE"

Most languages are judged, at least initially, by the support they provide for certain "key" issues. String manipulation is a typical example of such an issue.

Support for strings in C and Pascal marks two extremes of a spectrum. Pascal provides rather intuitive syntax for dealing with strings, while C provides for support via a powerful but non-intuitive library of functions. The advantage of standard C is the programmer's ability to easily create superior string handling functions at the cost of readability. The advantage of standard Pascal is the programmer's ability to create very readable code at the expense of flexibility.

C++ provides something between these C and Pascal. C++ doesn't have built-in support for strings, but the class structure and messaging protocol allows you to design string classes that are intuitive and easy to change. When this is done, you can use strings as if C++ has predefined definitions for what strings are and how to manipulate them. Furthermore, C++ lets you create small classes whose sole purpose is to aid in housekeeping chores or other code-level cleanup that is sometimes required when programming complex systems like the Macintosh.

HOUSEKEEPING À LA C++

General techniques One of the more tedious tasks facing Macintosh programmers is the necessity to save different states at various points in one's code. MacApp does nothing but add to the complexity of the problem. Lock state of handles, pen states, focused views, and permanent allocation flags are simple examples of things MacApp programmers need to keep track of and keep in sync with code.

The classic approach to this problem is to write code resembling the following:

{
    StateRecord aState;
    GetCurrentState(aState);
    //  Insert code here that screws around
    //  with the element we want to save
    SetCurrentState(aState);
}

This code is straightforward and works fine. However, it will run into problems when you need to contend with MacApp failure handling. In this situation, you need to provide failure handling code to restore the state appropriately, as follows:

{
    StateRecord         aState;
    FailInfo            fi;
    GetCurrentState(aState);
    if (fi.Try())
    {
        //  Insert code here that screws around
        //  with the element we want to save
        SetCurrentState(aState);
        fi.Success();
    }
    else
    {
        SetCurrentState(aState);
        fi.ReSignal();
    }
}

This code is functional and perfectly acceptable, though rather wordy. It uses a simple concept-it preserves the state across this section of code. This bit of housekeeping is a perfect candidate for an object.

The technique I describe here applies to all forms of housekeeping. But for sake of brevity, I'll restrict my explication to one application-locking a handle.

CHandleLocker
class CHandleLocker
{
private:
    Handle      fHandle;
    Boolean     fWasLocked;
    FailInfo    fFailInfo;
protected:
    pascal void HandleFailure(OSErr err, long message);
    virtual pascal RestoreLockState();
public:
    CHandleLocker(Handle theHandle);
    virtual ~CHandleLocker();
};

CHandleLocker is a simple class that assures a handle remains locked within a particular scope and that it reverts to its previous state outside that scope. Using CHandleLocker, you can write code that utilizes handle locking in the following manner:

{
    CHandleLocker   lockSomeHandle(someHandle);
    //  Insert code that relies on someHandle being locked
}

Magically, when this code block executes, someHandle will be locked. The intervening code will execute, and someHandle will be returned to the state it was in (either locked or unlocked) when the block began. As an added bonus, if a MacApp failure is raised somewhere in the intervening code, someHandle will still be left in its original state. Amazingly, CHandleLocker adds safe handle locking to C++, and gives it a syntax that is relatively simple and unobtrusive.

Under the hood of CHandleLocker

To accomplish this little bit of heaven, CHandleLocker relies on the constructor and destructor mechanisms of C++ classes. The constructor and destructor are the core of the class. The constructor is called automatically when an instance is created, as in the declaration of lockSomeHandle in the example above. The destructor is called when an instance is destroyed, as in the closing curly brace in the same example.
CHandleLocker::CHandleLocker(Handle theHandle)
{
    fHandle = theHandle;
    fWasLocked = IsHandleLocked(fHandle);
    if (!fWasLocked)
        HLock(fHandle);
    CatchFailures(fFailInfo, CHandleLocker::HandleFailure, this);
}

CHandleLocker::~CHandleLocker()
{
    Success(fFailInfo);
    this->RestoreLockState();
}

The constructor simply saves the handle and its original lock state in instance variables. If the object is originally unlocked, it locks it. Finally, a MacApp failure handler is installed. In the destructor, the failure handler is removed and the lock state is restored. The remaining parts of the class are the method to restore the lock state and the failure handler:

void CHandleLocker::RestoreLockState()
{
    if (!fWasLocked)
        HUnlock(fWasLocked);
}

pascal void CHandleLocker::HandleFailure(OSErr err, long message)
{
    this->RestoreLockState();
}

RestoreLockState is straightforward. It simply refers to the fWasLocked boolean instance variable and unlocks the handle if it was originally unlocked. But the failure handler is a bit more involved and will take a short explanation.

CHandleLocker::HandleFailure is mediated using the Pascal-style failure handling routines, CatchFailures and Success. Therefore, the failure handling routine does not need to resignal the failure since returning from the routine automatically invokes the next failure handler on the stack.

Also, the method is declared with the keyword pascal, indicating that we want the return values and arguments passed Pascal-style. This is important once you recognize that methods with Pascal-style arguments will have the metavariable this passed as the last "argument" of the routine, the same position the Pascal static link variables occupy (see the September installment of The Soup Kitchen for a discussion of nested procedures and static links). Notice that this is passed as the static link variable in the CatchFailures argument list above.

That's all there is to this simple little utility class. I challenge the reader to write utility objects to handle other housekeeping chores. To get started, look at the PenState of QuickDraw, and the focus information in the MacApp view architecture. It should be fairly simple to use CHandleLocker as a template to build a useful library of magic little objects.

STRINGS

Admittedly, utility classes are simple examples and do not come close to making the case for language extensibility. However, having opened the door and provided some decent groundwork, let's take a look at CStr255, a very nice piece of work from the MacApp Team at Apple.

Any Macintosh programmer is familiar with the basic Pascal string. The Pascal string differs from the C string in the following ways:

  • Pascal strings are limited to 255 characters; C strings are basically unlimited in length.
  • Pascal strings contain the length of the string in their first byte; C strings are terminated with a null byte (hex 00).
  • Pascal strings are supported with Pascal language syntax; C strings are supported by convention and a standard C library.

Pascal programmers are fortunate to be able to use standard Macintosh strings, Str255, with little or no effort. Since these strings are nothing more than real Pascal strings, the compiler and run time environment take care of the details. The C++ programmer has the ability to use Pascal-like syntax to define and manipulate Pascal strings, but must be constantly aware of the difference and must maintain discipline to convert to the appropriate format as necessary.

Unlike the example of maintaining the lock state of a handle, writing a fully capable language-extending class like CStr255 is beyond the skill of all but master C++ architects. CStr255 provides an excellent example of what C++ can do, but it should not be imitated by the novice. This warning is sorely lacking in all C++ books I've encountered and is the primary cause of problems with novice and intermediate level C++ programmers. To quote Barnum and Bailey, "Kids, don't try this at home."

In its final form, CStr255 allows you to write code that looks like this:

{
    CStr255 aString;
    aString = "A string";
    aString = aString + "another string";
    if (aString == anotherString)
        gApplication->Beep(3);
    theLength = aString.Length();
    theFifthCharacter = aString[5];
    DrawString(aString);
    DrawString("A string constant");
    printf("This line prints C strings %s", (char*)aString);
}

In short, you can write code that looks like Pascal but that integrates well with your C++ code.

Constructor magic

The first rabbit I'll pull out of CStr255 involves its constructors. The designers of CStr255 provide a constructor that takes a single char* as an argument: CStr255::CStr255(char*).

Since all standard C strings and string constants in C++ are of type char*, this means you can assign C strings and string constants directly to any variable of type CStr255. Furthermore, any routine that takes a CStr255 as an argument, like DrawString, can be passed a C string or string constant with no difficulty.

Similarly, CStr255 provides for constructors from other Macintosh string types (CStr63 for example), other C types (like unsigned char*), and even a constructor from a longint that is used to convert ResType and OSType variables to their string equivalents. As with all the CStr255 code I'll quote from here, these constructors are found in the file PascalString.h in the MacApp 3.0b3 release.

Operator magic

Constructors are handy and useful, but without the operators +, =, ==, and others, CStr255 would just be a neat utility class. Instead, with these operators, you have access to a truly powerful class that adds Pascal strings to the C++ language with a very simple and intuitive syntax. Since it's not reasonable to reproduce the CStr255 code here, I'll leave you with a taste of the features available for string handling and short examples of their utility.

Constructors

CStr255();                            //  CStr255 aStr
CStr255(char*);                       //  aStr = "A string"
CStr255(const CStr255&);          // aStr = anotherStr255
CStr255(const CStr63&);           //  aStr = aStr63
CStr255(long);                        //  aStr = 'PICT'

Operators

char& operator[](short pos);      //  aStr[4] = 'P'
CStr255 operator+(CStr255&);      //  aStr = bStr + cStr
CStr255 operator=(CStr255&);      //  aStr = bStr
Boolean operator==(CStr255&);     //  if (aStr = bStr)
Boolean operator==(const char*);      //  if (aStr == "A string")
Boolean operator>(const char*);    //  if (aStr > "ABC")

Handy functions

char& Length();                   //  aStr.Length() = 7
Boolean IsEmpty();                    //  if (aStr.IsEmpty())

Conversions

operator char*();                     //  printf("%s", aStr);
operator long();                      //  GetResource(long(aStr), kID);
 
AAPL
$118.93
Apple Inc.
-0.07
MSFT
$47.81
Microsoft Corpora
+0.06
GOOG
$541.83
Google Inc.
+1.46

MacTech Search:
Community Search:

Software Updates via MacUpdate

Adobe Photoshop Elements 13.0 - Consumer...
Adobe Photoshop Elements 12--the #1 selling consumer photo editing software--helps you edit pictures with powerful, easy-to-use options and share them via print, the web, Facebook, and more.Version... Read more
Skype 7.2.0.412 - Voice-over-internet ph...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
HoudahSpot 3.9.6 - Advanced file search...
HoudahSpot is a powerful file search tool built upon MacOS X Spotlight. Spotlight unleashed Create detailed queries to locate the exact file you need Narrow down searches. Zero in on files Save... Read more
RapidWeaver 6.0.3 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
iPhoto Library Manager 4.1.10 - Manage m...
iPhoto Library Manager lets you organize your photos into multiple iPhoto libraries. Separate your high school and college photos from your latest summer vacation pictures. Or keep some photo... Read more
iExplorer 3.5.1.9 - View and transfer al...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
MacUpdate Desktop 6.0.3 - Discover and i...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
SteerMouse 4.2.2 - Powerful third-party...
SteerMouse is an advanced driver for USB and Bluetooth mice. It also supports Apple Mighty Mouse very well. SteerMouse can assign various functions to buttons that Apple's software does not allow,... Read more
iMazing 1.1 - Complete iOS device manage...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
PopChar X 7.0 - Floating window shows av...
PopChar X helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to access any font's special characters. Expanded... Read more

Latest Forum Discussions

See All

Mystery Case Files: Dire Grove, Sacred G...
Mystery Case Files: Dire Grove, Sacred Grove HD Review By Jennifer Allen on November 28th, 2014 Our Rating: iPad Only App - Designed for the iPad A decent new installment for the popular Mystery Case Files series.   | Read more »
Castaway Paradise – Tips, Tricks, and St...
Ahoy there, castaways: Were you curious about our own thoughts regarding this pristine shipwreck? Check out our Castaway Paradise review! Castaway Paradise is out for iOS, finally giving mobile gamers the opportunity to enjoy the idyllic lifestyle... | Read more »
Castaway Paradise VIP Subs are on Sale f...
Castaway Paradise VIP Subs are on Sale for a Limited Time, and a Special Holiday Update is Coming Soon Posted by Rob Rich on November 28th, 2014 [ | Read more »
Primitive Review
Primitive Review By Jordan Minor on November 28th, 2014 Our Rating: :: FOLK ARTUniversal App - Designed for iPhone and iPad True to its name, Primitive is about as straightforward as runners get.   | Read more »
7 tips to get ahead of the competition i...
7 tips to get ahead of the competition in Dynasty of Dungeons Posted by Simon Reed on November 28th, 2014 [ permalink ] Playcrab has launched their action-packed new dungeon crawler, Dynasty of Dungeons, today. | Read more »
Master of Tea Kung Fu Review
Master of Tea Kung Fu Review By Jordan Minor on November 28th, 2014 Our Rating: :: ONE DROP RULESUniversal App - Designed for iPhone and iPad Master of Tea Kung Fu is a creative and complex caffeinated brawler.   | Read more »
Monster Strike Review
Monster Strike Review By Campbell Bird on November 28th, 2014 Our Rating: :: BILLIARD STRATEGYUniversal App - Designed for iPhone and iPad Collect monsters and battle by flinging them across the battlefield in this strangely... | Read more »
Proun+ Review
Proun+ Review By Jennifer Allen on November 28th, 2014 Our Rating: :: TWITCHY RACINGUniversal App - Designed for iPhone and iPad Twitchy racing aplenty in Proun+, an enjoyably tricky title.   | Read more »
Lucha Amigos (Games)
Lucha Amigos 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Forget Ninja Turtles, and meet Wrestlers Turtles! Crazier, Spicier and…Bouncier! Sling carapaces of 7 Luchadores to knock all... | Read more »
Record of Agarest War Zero (Games)
Record of Agarest War Zero 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: HyperDevbox Holiday Turkey Black Friday Special Pricing! To celebrate the opening of the holiday season HyperDevbox... | Read more »

Price Scanner via MacPrices.net

Up To 75% Off Infovole Text Apps Over Black F...
Infovole’s entire range of apps, including the Textkraft family of word processors for iPads and iPhones, is being offered at 50-75% off over the Black Friday and Cyber Monday weekend. The five-day... Read more
Black Friday: Up to $60 off Mac minis, NY tax...
 B&H Photo has new 2014 Mac minis on sale for up to $60 off MSRP as part of their Black Friday sale. Shipping is free, and B&H charges NY sales tax only: - 1.4GHz Mac mini: $449.99 $50 off... Read more
Black Friday: 27-inch 5K iMac for $2299, save...
 B&H Photo continues to offer Black Friday sale prices on the 27″ 3.5GHz 5K iMac, in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP... Read more
Karalux Announces 24K Gold-Plated iPhone 6
Karalux, a Vietnam-based jewellery firm, has launched a unique 24 karat gold-plated iPhone 6 version with gold-cast monolithic dragon on its back panel. The real 24 karat gold plated enclosure doesn’... Read more
Black Friday: 13-inch 2.6GHz Retina MacBook P...
 B&H Photo has lowered their price for the 13″ 2.6GHz/128GB Retina MacBook Pro to $1159 for Black Friday. That’s $140 off MSRP, and it’s the lowest price for this model (except for Apple’s $1099... Read more
View all the Black Friday sales on our Mac Pr...
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. View Black Friday sale prices at a... Read more
Black Friday: 11-inch MacBook Air for $779, s...
 Best Buy has lowered their price for the 2014 11″ 1.4GHz/128GB MacBook Air to $779.99 for Black Friday. That’s $120 off MSRP. Choose free shipping or free local store pickup (if available). Sale... Read more
Apple Store Black Friday sale for 2014: $100...
BLACK FRIDAY The Apple Store has posted their Black Friday deals for 2014. Receive a $100 PRODUCT(RED) branded iTunes gift card with the purchase of select Macs, $50 with iPads, and $25 with iPods,... Read more
Black Friday: 15% off iTunes Gift Cards
Staples is offering 15% off $50 and $100 iTunes Gift Cards on their online store as part of their Black Friday sale. Click here for more information. Shipping is free. Best Buy is offering $100... Read more
BEVL Releases Dock Tailored for iPhone 6 and...
Seattle based BEVL has released their first product: an iPhone dock that is divergent in build quality, rock-solid function and visual simplicity to complement the iPhone. BEVL is now accepting... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.